The chemical effects of ultrasound derive primarily from acoustic cavitation. Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of the liquid motion into heating of the contents of the bubble. The high local temperatures and pressures, combined with extraordinarily rapid cooling, provide a unique means for driving chemical reactions under extreme conditions. A diverse set of applications of ultrasound to enhance chemical reactivity has been explored with important uses in synthetic materials chemistry. For example, the sonochemical decomposition of volatile organometallic precursors in low-volatility solvents produces nanostructured materials in various forms with high catalytic activities. Nanostructured metals, alloys, oxides, carbides and sulfides, nanometer colloids, and nanostructured supported catalysts can all be prepared by this general route. Another important application of sonochemistry in materials chemistry has been the preparation of biomaterials, most notably protein microspheres. Such microspheres have a wide range of biomedical applications, including their use in echo contrast agents for sonography, magnetic resonance imaging, contrast enhancement, and oxygen or drug delivery. Other applications include the modification of polymers and polymer surfaces. 295 0084-6600/99/0801-0295$08.00 Annu. Rev. Mater. Sci. 1999.29:295-326. Downloaded from www.annualreviews.org by Indiana University -Purdue University Indianapolis -IUPUI on 09/25/12. For personal use only.

This article will begin with an introduction to acoustic cavitation, the physical phenomenon responsible for the chemical effects of ultrasound. Some recent applications of sonochemistry to the synthesis of nanophase and amorphous metals, as well as to heterogenous catalysis, will then be highlighted. Finally, we will examine the effects of ultrasound on metal powders in liquid-solid slurries.The chemical effects of ultrasound do not come from a direct interaction of sound with molecular species. Ultrasound has frequencies from around 15 kilohertz to tens of megahertz. In liquids, this means wavelengths from centimeters down to microns, which are not molecular dimensions. Instead, when sound passes through a liquid, the formation, growth, and implosive collapse of bubbles can occur, as depicted in Figure 1. This process is called acoustic cavitation.More specifically, sound passing through a liquid consists of expansion waves and compression waves. As sound passes through a liquid, if the expansion wave is intense enough (that is, if the sound is loud enough), it can pull the liquid apart and form a bubble (a cavity). The compression wave comes along and compresses this cavity, then another expansion wave re-expands it. So we have an oscillating bubble going back and forth, say, 20,000 times a second.

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Polymer stabilized nanodiscs are self-assembled structures composed of a polymer belt that wraps around a segment of lipid bilayer, and as such are capable of encapsulating membrane proteins directly from the cell membrane. To date, most studies on these nanodiscs have used poly(styrene- co-maleic acid) (SMA) with the term SMA-lipid particles (SMALPs) coined to describe them. In this study, we have determined the physical and thermodynamic properties of such nanodiscs made with two different SMA copolymers. These include a widely used and commercially available statistical poly(styrene- co-maleic acid) copolymer (coSMA) and a reversible addition-fragmentation chain transfer synthesized copolymer with narrow molecular weight distribution and alternating styrene and maleic acid groups with a polystyrene tail, (altSMA). We define phase diagrams for each polymer, and show that, regardless of polymer topological structure, self-assembly is driven by the free energy change associated with the polymers. We also show that nanodisc size is polymer dependent, but can be modified by varying polymer concentration. The thermal stability of each nanodisc type is similar, and both can effectively solubilize proteins from the E. coli membrane. These data show the potential for the development of different SMA polymers with controllable properties to produce nanodiscs that can be optimized for specific applications and will enable more optimized and widespread use of the SMA-based nanodiscs in membrane protein research.

Dibutyltin polymers with much higher molecular weights (GPC, polystyrene standards: M , = ca. 1 x 106) than previously reported are prepared by Wurtz coupling of BuzSnClz monomer; the reaction conditions are critical in achieving the maximum chain length, as polymer degradation results from prolonged reaction times.

Solute-induced quenching of sonoluminescence (SL) is reported for aqueous solutions of two homologous series of methyl esters and ketones using low (20 kHz) and high (515 kHz) ultrasound frequencies. SL data at 20 kHz from aqueous solutions containing alcohols and carboxylic acids are also presented to compare with previously published results at 515 kHz. In addition to supporting the previous findings on the existence of stable and transient bubbles at 515 and 20 kHz, respectively, the results suggest that the hydrogen-bonding characteristics of the solutes also play a major role in the extent of SL quenching. An increase in the SL intensity at low concentrations for most of the solutes suggests that these solutes increase the number of "active" bubbles by hindering the coalescence of bubbles. It is concluded that the effect of the solutes on the SL signal from aqueous solutions at both frequencies is primarily due to the balance of two factors, namely, the incorporation of solute within the bubble, leading to SL quenching, and the prevention of coalescence of the bubbles, leading to SL enhancement. At the higher frequency, SL quenching by the solutes is the main influence on the emission yield. However, at the lower frequency, hindrance to coalescence by the solutes dominates at lower concentrations and leads to SL enhancement. The implications of these results for optimizing conditions for aqueous sonochemical reactions are discussed.

The results of ultrasonic irradiation of polystyrene solutions in toluene, benzene and carbon tetrachloride are described. The effect of a range of conditions such as ultrasonic intensity, solution concentration and varying solvents were investigated. The non‐random nature of the degradation process was clearly demonstrated and the effect of different solvents was interpreted in terms of their thermodynamic properties.